1. Technical Field
The present invention relates to field effect transistors, and more particularly, to carbon-nanotube field effect transistors.
2. Discussion of Related Art
In the field of molecular nanoelectronics, few materials show as much promise as nanotubes, and in particular carbon nanotubes, which comprise hollow cylinders of graphite, angstroms in diameter. Nanotubes can be implemented in electronic devices such as diodes and transistors, depending on the nanotube's electrical characteristics. Nanotubes are unique for their size, shape, and physical properties. Structurally a carbon-nanotube resembles a hexagonal lattice of carbon rolled into a cylinder.
Besides exhibiting intriguing quantum behaviors at low temperature, carbon nanotubes exhibit at least two important characteristics: a nanotube can be either metallic or semiconductor depending on its chirality (i.e., conformational geometry). Metallic nanotubes can carry extremely large current densities with constant resistivity. Semiconducting nanotubes can be electrically switched on and off as field-effect transistors (FETs). The two types may be covalently joined (sharing electrons). These characteristics point to nanotubes as excellent materials for making nanometer-sized semiconductor circuits.
In addition, carbon nanotubes are one-dimensional electrical conductors, meaning that only one-dimensional quantum mechanical mode carries the current. This can be a significant advantage with respect to the device performance of a carbon-nanotube based transistor since scattering in the material is significantly suppressed. Less scattering means a better performance of the device.
For a three terminal device, such as an FET, a gate (the third terminal) needs to be isolated from the electrically active channel region as well as a source and a drain. For this purpose a dielectric material, e.g., silicon dioxide can be used. To improve device characteristics in silicon devices, the thickness of this layer can be reduced. This reduction increases the gate capacitance and improves the gate-to-channel coupling. For standard silicon field-effect devices the gate capacitance scales inversely proportional to the dielectric film thickness. For currently manufactured high-performance processors, the SiO2 thickness is less than 4 nm. Significantly, further reduction can be difficult to achieve since gate leakage through the dielectric film increases exponentially for an oxide thickness below 4 nm.
However, the gate capacitance for a carbon-nanotube transistor does not scale inversely proportional with the dielectric film thickness. Instead, carbon-nanotubes follow a logarithmic scaling law. In comparison with a standard silicon field-effect transistor, the gate capacitance for a carbon-nanotube transistor can be larger because of the cylindrical geometry of these objects.
No known system or method has implemented a nanotube to achieve performance and smaller size in an FET. Therefore, a need exists for a system and method of preparing nanotube based FETs.